The need to utilize short and narrow, austere landing fields, particularly for STOL aircraft, requires accurate glidepath and directional control to limit dispersion on landing. While airfield performance is improved with lower speed operation, the handling qualities and controllability of STOL aircraft diminish significantly at the slower approach speeds experienced during a STOL landing.
Upon certain conditions of aircraft operation, the relatively streamline airflow across the upper surface of the wing can become partially or substantially separated from the upper surface thereof, resulting in a substantial loss of control. This flow separation, or stall condition, typically occurs with an aircraft wing at relatively low flying speeds found, for example, at landing and takeoff conditions when the wing is operated at a relatively high angle of attack and when maximum lift generation is particularly critical. This loss of control, resulting from the above-noted stall condition, is also applicable to aerodynamic stability and control surfaces, such as, for example, ailerons, rudders, and elevators.
Airflow separation from an airfoil is a particularly onerous problem when designing an aircraft for STOL operation. Because large pitching moments and engine-out yawing and rolling moments are associated with known powered-lift arrangements in the STOL mode, the conventional aircraft, configured for STOL performance, requires large control surfaces. These large control surfaces result in a drag penalty at cruise. Furthermore, even with the use of large control surfaces, the airspeed needed for minimum control is relatively high compared to that which is theoretically possible with the optimum powered-lift arrangements.
In order to provide a pilot with a greater lateral response for a given control input at STOL operation and thus reduce the time required for improving lateral tracking errors, enhanced sideforce controls are necessary. Known solutions include thrust deflection vanes positioned in the slipstream as well as the deflection of ailerons and rudders having large control surfaces as previously noted.
One problem with the movable vane configuration is large thrust losses. Additionally, the movable vanes require complex actuators which are difficult to maintain, thus providing a reduced reliability of performance.
As noted above, the use of ailerons and rudders having large control surfaces has the problem of resulting in a drag penalty at cruise. Further, aileron and rudder deflections induce yawing and rolling moments which are quite undesirable near the ground and can result in contact with obstacles and the loss of the aircraft.